The present invention concerns protective relays for power systems, and more particularly concerns a multifunction relay used primarily to protect feeder lines in a power system.
FIG. 1 shows the conventional approach to protecting feeder line portions of a power system. A plurality of feeder lines extend from a distribution bus, with laterals extending off each feeder line to the individual customers. Each feeder line has its own circuit breaker. In FIG. 1, a distribution bus 10 is connected to the secondary of a power system transformer (not shown), with the transformer secondary typically grounded-wye. Extending from the distribution bus 10 are the feeder lines, with feeder lines 12 and 14 being shown in FIG. 1. Each feeder, i.e. feeder 12, has, as indicated above, a circuit breaker 16 associated therewith, along with four protective relays, 20-23, which receive the power signal current through current transformers (CTs) on the feeder line. There is typically one relay for each phase of the power signal (A,B,C) and one (N) for the residual current.
Each of the four relays 20-23 will include one or perhaps two relay elements. A two-element relay will typically include one element which operates instantaneously for high overcurrent conditions, while the other element operates with a time delay depending upon current level. In the arrangement of FIG. 1, any one element in any relay 20-23 can trip circuit breaker 16. The above scheme provides redundancy for any phase-to-phase fault, e.g. an AB fault, since the fault current flows through two separate relays. Three-phase (phase-to-phase-to-phase) faults are also redundantly protected (three separate relays). The conventional arrangement of FIG. 1 is still in wide use because it clearly provides two results which have historically been very important to the power engineer, namely, specific redundant protection (if one relay goes out, there is a direct backup) and relay-line autonomy, i.e. each protective relay is associated with a particular line.
The conventional scheme has been traditionally favored even though it does have several disadvantagesxe2x80x94it is typically quite expensive, there are a large number of relays to mount and connect, there is not a true redundancy for the residual current relay, and there are no support functions provided, i.e. event reporting, reclosing, etc.
FIG. 2 shows a more current approach to protecting feeder lines using a single multifunction relay (MFR), with one relay for each feeder line. In this approach, a single relay, i.e. relay 28, provides overcurrent protection for each phase of the power signal current and the residual current on a feeder line 29. Relay 28 controls a circuit breaker 30. Multifunction relays are computer-based, in which the functions of the previous individual relay elements are implemented in software in a microprocessor. This approach is desirable since it retains the desired feature of relay-feeder line autonomy, is capable of providing supporting information and functions, including event reporting and reclosing, and is less expensive generally and easier to install.
However, the multifunction relay approach has been questioned because of a lack of apparent reliability (i.e. redundancy), although the relay typically has a self-test alarm feature so that if the relay does in fact fail, an alarm contact will close and the relay can be conveniently replaced. Further, another relay, referred to as a bus backup relay 32, has been used upstream of the feeder lines, controlling circuit breaker 34 for the bus 35, and is designed to trip the breaker 34 if a fault should occur on a feeder line where the feeder relay has failed. Relay 32 operates off the secondary of the distribution line transformer 36. The multifunction relay approach of FIG. 2 does achieve cost savings and has achieved considerable acceptance in the industry.
In another attempt, which has never been commercially implemented, a single computer-controlled system was used to provide protection for a large number of individual feeder lines, i.e. all of the feeders in a particular power substation. This system was experimental but did not work and in any event was considered to be fundamentally unacceptable because of a lack of reliability (the failure of the computer meant loss of all protection for all the feeder lines) and because the protection was basically too integrated, i.e. there was no separate autonomy (correlation) between a given relay and a specific feeder line. Thus, the industry has to date steadfastly maintained a one-to-one relay-to-feeder approach.
Accordingly, the present invention is a dual protective relay for protecting power systems, comprising: means for obtaining current signals for each of three phases of power signals from at least two power line sources thereof, each source having associated therewith a circuit breaker system for interrupting power on the power line associated therewith; a single processing means for processing said current signals relative to selected threshold values; and means for operating the circuit breaker system for the particular power line source when the processing means indicates a fault condition thereon.